Separation of xylene isomers



March 14, l1967 Dl A McCAULAY SEPARATION OF XYLENE ISOMERS Filed April 18. 1963 mmm* EmE absl T TUR/VE Y United States Patent O 3,309,414 SEPARATION F XYLENE ISOMERS David A; `McCaulay, Homewood, Ill., assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana Filed Apr. 18, 1963, Ser. No. 273,889 3 Claims. (Cl. 260-674) This invention relates to a process for the selective separation or concentration of meta-xylene from its aro- 4operating costs.

In the process of the invention, a feed containing metaxylene admixed with at least one other C8 aromatic is c-ontacted in a first extraction zone with a complex formed from HF, BF3 and methylnaphthalenes. In the extraction zone, the meta-xylene displaces the methylnaphthalenes from the complex, and there is withdrawn from the extraction zone an extract (acid-phase) comprising a complex of HF, BF3 and rneta-xylene. The extract is then charged to a second extraction zone wherein it is contacted with methylnaphthalenes. The product metaxylene is withdrawn as the raffinate phase from the second extraction zone and has a concentration of metaxylene (that is, a meta-xylene purity) greater than that of the feedstock. The extract phase withdrawn from the second extraction zone comprises an HF-BFa-methylnaphthalenes complex and is advantageously used as the extraction agent in the first extraction zone. The methylnaphthalenes in the raffinate from the first extraction zone may be recovered therefrom, usually by distillation,` and charged to the second extraction zone.

An isomerization procedure may be combined with the separation process broadly outlined in the preceding paragraph. Such an isomerization procedure may be conducted, for instance, according to the disclosure of D. A. McCaulay and A. P. Lien in U.S. Patent No. 2,662,925. The feed for the isomerization procedure may be the C8 aromatics separated Afrom the rafinate from the first extraction zone, or it may be a portion of the acid layer phase removed from the first extraction zone as hereinafter described in greater detail. Y

The feedstocks used in the process are hydrocarbon fractions containing meta-xylene and at least one other aromatic isomer thereof, i.e., ortho-xylene, para-xylene or ethyl benzene. Generally, the feedstocks will contain more than one such isomer. The meta-xylene content may vary over a wide range, from as low as up to 95%. A typical C8 aromaticfeedstock from naphtha or reformate contains about 50% meta-xylene, 20% each of orthoand para-xylene and 10% ethyl benzene. It is desirable that the ethyl benzene concentration Vin the feed be low, inasmuch as ethyl benzene tends to disproportionate in the process to benzene, which is Withdrawn from the first extraction zone in the raffinatephase, andv to either diethyl benzene or ethyl dimethyl benzenes, which appear in the extract phase from the first extraction zone. In addition, non-aromatic hydrocarbons may also be in the feed, and are withdrawn from the first extraction zone in the raiinate phase. It is desirable to minimize and preferably reduce to less than 0.1% water or sulfur compounds in the feed, because such compounds complex with BFS, and, furthermore, because of corrosion problems. v v v 35,309,414 Patented Mar. 14, 1967 ICC In performing the process, the feedstock is contacted with a complex of HF-BF3-methylnaphthalene. When initially preparing the complex, either the alphaor betamethylna-phthalene isomer 'may be used, although the alpha-isomer is preferred because of its lower melting point.` During the practice of the process, the methylnaphthalene is isomerized to an approximate ratio of 30 parts of alpha-methylnaphthalene to 70 parts of betamethylnaphthalene.

Sufficient HF should be used to provide from about 5 to about moles thereof per mole of methylnaphthalene, preferably about 10 to 30 moles HF per mole of methylnaphthalene. Sufiicient BF3 is provided to give in the range of about 0.8 to about 1.2 moles thereof per mole of methylnaphthalene, preferably about 0.95 to 1.1 moles and advantageously a mole ratio of one. Excessive amounts of BF3 relative to the methylnaphthalenes are undesirable, because excess BF, will complex with the meta-xylene preferentially to displacement by meta-xylene of the methylnaphthalene from the HF-BF3-methylnaphthalene complex.

The HF-BFa-methylnaphthalene complex is preferably .prepared by adding the desired amount of methylnaphthalene to liquid HF with stirring. The desired amount of BF3 is then added. Alternatively, the complex may be prepared by adding methylnaphthalene and BF3 to liquid HF simultaneously or in alternating increments. The complex may be so prepared at atmospheric pressure. The preparation reaction is exothermic, and appropriate cooling means are usually provided. A complex may also be formed by mixing together BF, and methylnaphthalene, and then adding liquid HF, or -byv adding first BPB and then methylnaphthalene to the HF, but with these two alternative procedures high pressure is used to confine the BFa.

The contacting in the first extraction zone of the feedstock with the HF-BF3-methylnaphthalene complex may be done in lconventional liquid-liquid extraction equipment of a character and operated in accordance with techniques described in Perrys Chemical Engineers Handbook, starting at page 713 (3rd edition, 1950), Kirk- Othmers Encyclopedia of Chemical Technology, volume 6, pages 120-140 (1st edition, copyright 1951) and in U.S. Patent No. 2,528,892. The first extraction zone is operated at temperatures in the range of about 0 to 125 F., advantageously at 60-100 F. The extraction zone may be located out of `doors if desired, and, inasmuch as y y Such an extraction zone is normally operated in counter-.

. tion zone.

the extractionproduces substantially no heat of reaction, no facilities for cooling or heating the extraction zone are necessary. Sufficient pressure, preferably 5 to 50 p.s.i.g., should be used to maintain a liquid phase in the extrac- Additional pressure may be used without technological disadvantage in operating the process.

The number of extraction stages in the first extraction zone depends upon the purity desired in the meta-xylene product. The solvent extraction separation factor (beta, calculated as the mole lfraction of meta-xylene in the acid layer times the mole fraction of ortho-xylene plus paraxylene in the raffinate divided by the product of the mole fraction of meta-xylene in the raffinate times the mole fraction of ortho-xylene and para-xylene in the acid layer) in the first extraction zone is about 7-8. Accordingly, although one theoretical extraction stage yields an extract having about 74% meta-xylene, when starting with a feed having 50% meta-xylene and 50% of orthoplus para-xylene, it is generally preferred to use a plurality of extraction stages in the first extraction zone, such as 10 to 60 stages, and preferably 40-50 stages, in order to yield a meta-xylene product of 99% or greater with a product recovery in excess of and to displace substantially all the methylnaphthalene from the complex.

current fashion, with the HF-BFa-methylnaphthalene complex being introduced at its upper end, raffinate withdrawn at the upper end, and extract withdrawn at the bottom of the extraction zone. The xylene-rich feed is fed to the extraction zone at an intermediate point between top and bottom depending upon the composition of the feed. It is often advantageous to introduce into the bottom of the extraction zone a recycle stream of purified metaxylene in order to increase the purity of the final product for an extraction zone of a specified number of extraction stages.

The raliinate withdrawn from the top of the extraction zone is the hydrocarbon layer and is composed of orthoxylene, para-xylene, ethyl benzene, benzene (which may have res-ulted from the disproportionation of the ethyl benzene), non-aromatic hydrocarbons in the feed and methylnaphthalenes. In a continuous commercial plant, the raffinate is usually distilled to recover the methylnaphthalenes for use in the second extraction Zone. The distillation is easily performed to recover a methylnaphthalene bottom substantially free of C8 aromatics and lower boiling materials on account of the wide differential in boiling points between methylnaphthalenes and C8 aromatics.

The extract, which is the yacid layer phase, withdrawn from the first extraction zone contains an HF-BF3-metaxylene complex, formed by the displacement by metaxylene of the methylnaphthalenes in the feed complex. The first zone extract is -contacted in a second extraction Zone with methylnaphthalenes. Suiiicient methylnaphthalenes are used to provide in the ran-ge of about 0.8 to 1.2 moles thereof per mole of the HF-BFg-meta-xylene complex, preferably Within a mole ratio of one. The second extraction zone is operated at temperatures and pressures comparable to those referred to above in respect of the first extraction Zone, and may differ from those actually used in the first zone. The extraction separation factor (beta) between methylnaphthalenes and meta-xylene is about 5.3 for alpha-methylnaphthalene 'and 7.5 for betamet'hylnaphthalene. Accordingly, the second extraction Zone lwill generally be designed to have to 15 extraction stages, depending upon the extent of recovery and pun'ty desired in the product meta-xylene.

The extract of the second extraction Zone is a complex of HF-BFS-methylnaphthalenes which is normally used as the extracting agent in the first extraction zone. A minor amount, about l to 10%, of the extract is discarded from the system to minimize the build-up of tars and other high molecular weight material. Additional amounts of fresh HF-BFa-methylnaphthalene complex are 'added as a replacement for the material withdrawn.

The raiiinate of the second extraction zone is metaxylene. The raffinate may contain small amounts of methylnaphthalene and any diethyl benzenes or ethyl-xylenes formed by disproportionation of any ethyl benzene in the feed. These impurities may be readily removed from the produ-ct meta-xylene by distillation. Small amounts of HF and BF3 may also be dissolved in the rainate and may be removed therefrom by distillation or caustic wash.

The practice of the process may be further illustrated by reference to the figure, which is a schematic flow sheet. In describing the flow sheet, flow rates will be given in pounds per hour.

A feed containing 400 pounds meta-xylene, 200 pounds each of orthoand para-xylenes, and 100 pounds each of ethyl benzene and non-aromatics, is charged through line 11 into a first extraction column 12 having 45 extraction stages. An HF-BFS-methylnaphthalenes complex containing 2340 pounds of HF, 1170 pounds of BF3 and 2340 pounds of methylnaphthalenes is introduced near the top of column 12 through line 13 and passes downward through the column and countercurrently to the rising hydrocarbon layer. A recycle stream of purified metaxylene from line 25 is introduced near the bottom of column 12. The column is operated at 80 F. and 30 p.s.1.g.

From the top of column 12, a raffinate is withdrawn through line 14. The rafiinate contains most of the orthoand para-xylenes, benzene, ethyl benzene and methylnaphthalene. The raliin-ate is distilled in distillation column 15, from which a Cs-fraction is taken overhead through line 16 and a methylnaphthalene fraction of 99.8% pwrly withdrawn as bottoms product through line 17. 99.8% purity withdrawn `as bottoms product through line 18. The extract phase is a complex of HF-BFs-meta. xylene and also contains small amounts of ethyl-xylenes and diethyl benzene. The extract in line 18 is introduced into the top of a second extraction column 19 which has 10 extraction stages. The recovered methylnaphthalenes from distillation tower 15 are introduced through line 17 into the bottom of second extraction column 19, and rise through the column countercurrently to the descending complex which was introduced into the top of the column through line 18.

A raflinate layer is withdrawn from the top of the second extraction column through line 20. The -raiiinate s introduced into a caustic wash zone 21 to which is fed through line 22 an aqueous caustic solution. The metaxylene stream is -withdrawn through line 23 and the discarded caustic wash removed through line 24. A recycle cut of raflinate inline 20 is withdrawn through line 25 for use -as aforesaid in column 12. Alternatively, such recycle cut may be taken from hereinafter described line 28 The meta-xylene in line 23 is charged to distillation tower 26, in which any methylnaphthalenes or other aromatics boiling above meta-xylene, such as diethyl benzene or ethyl xylenes, are removed through line 27 as a bottoms product and discarded. The finished product metaxylene is taken overhead through line 28, and has a purity of 99% at a yield of 96% based on meta-xylene in the feed.

The HF-BF3-methylnaphthalene complex which is the extract phase is removed from column 19 through line 29. A small portion of the extract is continuously or intermittently withdrawn through line 30 and discarded in order to prevent the build-up of undesirable contaminants in the complex. The remainder of the extract is returned to the top of the iirst extraction column 12 through line 13.

From time to time, fresh HF-BFa-methylnaphthalene complex is added to the system through line 31.

The process described may be used in conjunction with the type of isomerization of xylenes disclosed in U.S. Patent No. 2,662,925. In combining xylene isomerization with the meta-xylene separation process disclosed, a side stream is withdrawn from the first extraction column 12 at a point intermediate between the point where the feed is introduced, Which is preferably near the bottom of the column in this embodiment, and the top of the column. The side stream is passed through a settler to separate the hydrocarbon phase, which is returned to the column, from the acid layer phase. This latter phase is then mixed With the extract withdrawn from the bottom of the rst extraction column. Suflicient additional BF3 is added under pressure to the mixture to raise the BF3 to total xylenes mole ratio to about 2. The resulting liquid mixture is then stirred for 5 minutes to about an hour at a temperature of about -180 F. to isomerize substantially all of the dimethyl benzenes to meta-xylene. Thereafter, the EP3 in excess of one mole per mole of metaxylene is removed, leaving an HF-BF3-meta-xylene complex which is then contacted with methylnaphthalenes in the second extraction column 19 as previously described. In practicing the embodiment described in this paragraph, it is generally preferable not to recycle separated metaxylenes back to lirst extraction column 12.

Having thus described the invention, what is claimed is:

1. A process for separating meta-Xylene from a mixture of C8 aromatics whi-ch process comprises charging to a first extraction zone a feed 4comprising meta-xylene admixed with at least one other C8 aromatic and a complex of HF-BF3-methylnaphthalenes maintaining a temperature of 0 to 125 F. in the first extraction zone, withdrawing from said extraction zone an extract comprising a complex of HF-BF3-rneta-xylene, charging methylnaphthalenes `and said extract to a second extraction zone and maintaining a temperature of 0 to 125 F. in the second extraction zone, and removing therefrom a rainate comprising meta-xylene having a meta-xylene content greater than that of said feed.

2. The process of claim 1 wherein said HF-BF3-methylnaphthalenes complex is formed from alpha-methylnaphthalene.

3. A process for concentrating meta-xylene which comprises charging to a first extraction zone a complex of HF-BF3-methylnaphthalenes and a feed Icomprising metaxylene admixed with at least one other C8 aromatic maintaining a temperature of 0 to 125 F. in the first extra-ction zone, removing from said first zone a rafnate comprising methylnaphthalenes and a C8 aromatic, separating methylnaphthalenes from raffinate, removing from said rst Zone an extract comprising a complex of HF- BFg-meta-xylene, contacting said extract in a second extraction zone countercurrently with methylnaphthalenes References Cited by the Examiner UNITED STATES PATENTS 2,528,892 11/1950 Lien et al. 260-674 2,662,925 12/ 1953 McCaulay et al 260-674 2,727,078 12/ 1955 Shoemaker 260-674 2,728,803 12/1955 McCaulay et al 260-674 2,73 8,372 3/1956 Talbot 260-674 2,773,916 12/1956 Thorne et al 260-674 2,803,686 8/1957 Stein 260-674 2,852,575 9/1958 Lien et al. 260-674 3,151,177 9/1964 Hengstebeck et a1. 260-674 3,155,738 11/1964 Suld 260-674 DELBERT E. GANTZ, Primary Examiner.

C. E. SPRESSER, Assistant Examiner. 

1. A PROCESS FOR SEPARATING META-XYLENE FROM A MIXTURE OF C8 AROMATICS WHICH PROCESS COMPRISES CHARGING TO A FIRST EXTRATION ZONE A FEED COMPRISING META-XYLENE ADMIXED WITH AT LEAST ONE OTHER C8 AROMATIC AND A COMPLEX OF HF-BF3-METHYLNAPTHALENES MAINTAINING A TEMPERATURE OF 0 TO 125*F. IN THE FIRST EXTRACTION ZONE, WITHDRAWING FROM SAID EXTRATION ZONE AN EXTRACT COMPRISING A COMPLEX OF HF-BF3-META-XYLENE, CHARGING METHYLNAPHTHALENES AND SAID EXTRACT TO A SECOND EXTRACTION ZONE AND MAINTAINING A TEMPERATURE OF 0 TO 125*F. IN THE SECOND EXTRACTION ZONE, AND REMOVING THEREFROM A RAFFINATE COMPRISING META-XYLENE HAVING A META-XYLENE CONTENT GREATER THAN THAT OF SAID FEED. 